Hi PE, patents and other literature regarding subbing discuss gelatin dispersing in weak acids. For example, from US Patent 2461475, Kaszuba et al:

Page 1, paragraph 2 (I'm editing this a little for brevity)
... Since hardened gelatin, as used for glass plates does not adhere to a film base, it is necessary to use a mixture of gelatin with an organic solvent for the material of the film base, such as acetone .... or an alcohol/acetone mixture. Gelatin is insoluble in these organic solvents and the film base is insoluble in water, but it is possible to prepare dispersions consisting of gelatin in a mixture of one of the organic solvents and water with a volatile organic acid, such as acetic acid ... as a dispersing agent.
The rest of this patent goes on to discuss the idea of using ascorbic acid rather than acetic acid as the dispersing agent. I recently reread this patent and I don't think I am clear in what they mean by dispersion. Since acid serves to desensitize an emulsion, what role is played by the acid? Why not just swell and melt gelatin in water, then add to an acetone and methanol mixture?

Typically an 8% or so gelatin - water mixture sitting on the shelf in my darkroom at room temperature (about 70F) will gel after a few hours. The gelatin - vinegar mixture does not.

By the way, the subbing solution discussed in the patent does actually work on CTA but its very finicky to make. If you aren't careful and have the methanol at the right temperature, you get a white, sticky glop in the bottom of your container.

-- Jason